swift234

Generate the required AST-level thunk for subscript setters.

An Objective-C subscript setter has a type of the form 

  (this) -> (value, index) -> ()

while a Swift subscript setter has a type of the form

  (this) -> (index) (value) -> ()

Introduce a Swift "thunk" with the latter signature that simply calls
the underlying Objective-C method, and make sure that thunk gets
type-checked and IR-generated appropriately.


Swift SVN r3382